The role of the kidney in the homeostatic control of electrolytes, water and acid-base balance and the derangements that occur in certain pathologic states will be investigated. Metabolic balance, clearance, micropuncture and microperfusion techniques will be utilized in several different species of experimental animals (dogs, rats, rabbits, hamsters, psammomys) to investigate the following problems: H plus secretion---the mechanism of H plus secretion by the tubule, physiologic factors controlling its rate, and nature of the proximal defect in fructuose-induced renal tubular acidosis; proximal NaCl reabsorption and its relation to H plus secretion and reasorption of glucose, phosphate and amino acids; the role of extracellular volume and intrarenal physical factors in regulating distal sodium reabsorption; passive model of renal medulla countercurrent system in species other than rabbit.